EFFECT OF THE COOLING RATE AFTER SEMI-SOLID PROCESSING ON THE RESULTING MICROSTRUCTURE IN HIGH-CHROMIUM TOOL STEEL

Sharp-edged carbides in high-alloy tool steels reduce their toughness appreciably. Since they form during solidification, they are virtually impossible to remove by conventional routes. One of the techniques available to meet this purpose involves thermomechanical treatment with transition through the semi-solid state. It relies on using temperatures above the solidus, thereby re-dissolving the carbides in the austenitic matrix. The resulting structure consists of polyhedral austenite grains embedded in lamellar austenite-carbide network. Using appropriate forming parameters, this network can be removed by thermomechanical treatment. An experiment was carried out on X210Cr12 steel, combining semi-solid processing and subsequent thermomechanical treatment in a press. Hot plastic deformation led to re-dissolution of carbides from the eutectic network to the matrix. The rate of cooling after deformation played a major role in the evolution of the resulting microstructure. It was found that by varying the cooling rate one can obtain a broad range of hardness values. Water quenching led to fine-grained structures consisting of the M-A constituent and fine chromium carbides whose hardness exceeded 860 HV10. By contrast, very slow cooling caused pearlite to form, producing structures with a hardness of 230 HV10. The process represents an alternative to known and well established treatment routes which are normally used for tool steels with chromium carbides.